1. Field of the Invention
The present invention relates generally to the field of test measurement and more particularly to friction mensuration devices and most particularly to tribometers.
2. Background Art
Over one million people experience slip, trip, or fall accidents each year and approximately 17,000 die from the resultant fall injuries, second only to the number of automobile accident fatalities. These accidents are particularly serious for the elderly, who may suffer a fall injury that severely impairs their mobility, independence, and general health. These accidents can and do occur in a wide range of environments, including construction sites, private residences, playgrounds and recreational facilities, industrial settings, public and private property (e.g., sidewalks, highways/roadways, streets, parking lots, markets); public transportation facilities (airports, subways, train & bus stations) and facilities for the elderly and disabled. Hence, a great deal of attention is placed on the quality of walkway surfaces used for ambulation by pedestrians.
The criteria for walkway surface safety are determined by consensus of a panel of safety experts, based on measurements of the coefficient of static friction (SCOF) of surfaces under scrutiny. In general, physicists define the SCOF as the ratio of the maximum tangential force needed to cause a standard surface to slip divided by the force holding the surfaces in contact, herein called the normal force. A value of 0.5 for SCOF has been adopted as a threshold of safety for a walkway surface.
It is believed that the most representative measurements of a surface are taken in the field, that is, at the location of the surface to be tested. A large number of friction-measuring field devices (herein, tribometers) are used to perform friction measurements. Each type of field device employs a different effector mechanism for tribologic mensuration. Examples of tribometers include a variable incidence tribometer (VIT), a variable angle tribometer (VAT), a portable inclinable articulated strut slip tester (PIAST), and a family of articulated strut tribometers known in the art as Brunbager Mk. I slipmeter, Brunbager Mk. II slipmeter, and Brunbager Mk. III slipmeter, named after the device developer. An archetypal impact device is the “James” machine, is a bench mounted device, suitable for the laboratory and not the field. Each of the aforementioned tribometers may be characterized as an “impact”-type device, in which the parameter measured results from an impact between a test surface and a material sample affixed to the particular tribometer. Similar devices, such as the Sigler Pendulum or the British Pendulum, employ principles of the Conservation of Momentum to measure the kinetic forces corresponding to pendulum motion being arrested by a test material.
Each type of impact device is operator dependent, that is, the criteria for the critical onset of motion during testing cannot be uniquely determined. In addition, the “impact” aspect of these devices implies that a kinetic phenomenon is being used to determine a static phenomenon, namely the static coefficient of friction. Application of the First Law of Thermodynamics to the mechanics of impact devices shows that these devices dissipate some amount of kinetic energy during mensuration, causing wide variability in the measured value of “static” friction. Thus, a measurement reported by an impact-type device is an estimated proxy measurement for the static frictional force and, by extension, for SCOF. Not unsurprisingly, impact tribometers tend to generate non-reproducible, divergent numerical values for the same surface and condition under test. In fact, “impact devices” that rely on gravity, on spring loaded pressure, or on pneumatic pressure are actually energy converters that tend not to reliably produce their theoretically expected horizontal and vertical force components. In addition to the impact-type tribometer, a “pull”-type tribometer also is in use. Two devices exemplary of this type are known to ones of ordinary skill in the art as a horizontal pull slipmeter, and a horizontal dynamometer pull-meter. Each device relies on an operator applying a horizontal force on the slipmeter test device, which is coupled to the test sample. The point of motion initiation being taken as corresponding to the SCOF. Some “slipmeters” are closer in physical characteristics to spring-loaded fishing or laboratory scales. Again, an operator applies a horizontal force and notes the point motion begins. Variations induced by the operator are inevitable. The subjective and objective variability of device operation, unintentional or unavoidable application of non-horizontal force, and vagaries in result interpretation, can lead to a myriad of results for the same test conditions. Moreover, each of the “pull”-type devices use a respective test material (neolite, neoprene/rubber) that is generally unrepresentative of materials used by an actual pedestrian. A difficulty in reported and published measurements derived from impact and pull devices alike is that the devices produce divergent numerical values for their output. In 2005, testimony at an OSHA hearing revealed there was no test procedure or instrument that accurately correlates to the actual human walking and working condition. Thereafter, ASTM International (formerly, American Society for Testing and Materials) withdrew International Certification for all field measurement protocols for Coefficient of Friction, with no replacements. Hence, there is a need for a tribometer that provides a direct measurement for static friction force, which is operator-independent, and which produces results that are reliable, reproducible, and verifiable.